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Solving the Power/Energy Paradox for High-Power Defense Applications

Saturday, 01 December 2012

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While often used interchangeably,
power and energy are not the
same. Energy is the capacity to
perform and power is the rate at which
that capacity is exercised — the rate at
which energy is delivered. When measured,
energy is an amount; power is a
ratio. In relation to energy storage, a
common misconception is that the larger
the quantity of stored energy, the
higher the potential rate of power. In
fact, the opposite is true, rendering the
following principles wholly counterintuitive
to those unfamiliar with the
nuanced mechanics of energy storage:
when less energy is stored, a system is
capable of producing high power and
conversely, when more energy is stored,
a system is capable only of producing
low power. As a result, engineers are presented
with a challenge in the manufacture
of energy storage systems for highpower
mobile applications with the
highest energy density possible. In pursuit
of this goal, modern research into
lithium ion (Li-ion) electrochemistries
is proving very promising.

Li-ion Technology

As is widely known, use of Li-ion batteries
in the consumer electronics sector
is flourishing. This is also the technology
of choice for electric vehicle (EV) as well
as hybrid/plug-in hybrid electric vehicle
(HEV/PHEV) applications, mainly due
to its high energy density. By applying
the same Li-ion electrochemistry on the
industrial level, Saft America, Space and
Defense Division (Saft SDD) is making
significant headway in the quest to solve
the power/energy paradox — a development
that has been critical to a variety of
defense and space applications that
require very high power. This innovation
is being achieved by optimizing both the
electrochemical part of the cell as well as
the cell’s mechanical design.

A battery is an electrochemical energy
storage and conversion device, where the
inter-conversion between chemical and
electrical energy occurs via electron transfer
reactions, or oxidation-reduction
(redox) reactions. There are several classifications
of batteries — primary, secondary
rechargeable, mechanical replaceable,
reserve and thermal. The Li-ion battery is
classified as a secondary rechargeable battery.
In Li-ion battery cells, the charge
transfer between the positive electrode
(cathode) and negative electrode (anode)
is carried out by lithium ions in the electrolyte.
No metallic Li is used in a Li-ion
battery. Some of the commercial production
cathode materials include LiCoO2,
Li(NiCoAl)O2 (NCA), Li(NiCoMn)O2
(NMC), LiMn2O4 (LMO), and LiFePO4
(LFP). Currently, the most common
anode material is graphite. During cell discharge,
lithium ions are extracted from
the anode and inserted into the cathode.
The movement of Li-ion is carried out in
reverse during cell charge. The electrolyte
for a Li-ion battery is organic, which can
be stable at voltage exceeding three volts.

In the constant search for higher
power and capacity, researchers have
been focusing on groups of electrode
materials that can provide higher voltage
and/or higher capacity. For the
anode, silicon (Si) alloys — which can
potentially achieve 6x the capacity of
graphite — are actively pursued with the
goal of overcoming material stability and
cycle life issues. In the field of advanced
cathode materials, higher voltage phosphates
such as LiMnPO4 and LiCoPO4,
as well as high voltage spinel, e.g.
LiMnxNi2-xO4, are of research interest for improvement in material stability
and cycleability.

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